1. Field of the Invention
This invention relates in general to electric submersible pumps (ESPs) and, in particular, to systems, methods, and apparatuses for vibration attenuation within an ESP motor.
2. Brief Description of Related Art
Electric submersible pump (ESP) assemblies are disposed within wellbores and operate immersed in wellbore fluids. ESP assemblies generally include a pump portion and a motor portion. Generally, the motor portion is downhole from the pump portion, and a rotatable shaft connects the motor and the pump. The rotatable shaft may be one or more shafts operationally coupled together. The motor rotates the shaft that, in turn, rotates components within the pump to lift fluid through a production tubing string to the surface. ESP assemblies may also include one or more seal sections coupled to the shaft between the motor and pump. In some embodiments, the seal section connects the motor shaft to the pump intake shaft. Some ESP assemblies include one or more gas separators. The gas separators couple to the shaft at the pump intake and separate gas from the wellbore fluid prior to the entry of the fluid into the pump.
ESP motors generally include a stator and a rotor. The rotor may be positioned within the stator and may rotate on bearings. Generally, an electric current is applied to the stator to generate a rotating magnetic field. The rotating magnetic field induces a current to flow through the rotor that results in toque or rotation of the rotor relative to the stator. Rotors and stators are generally cylindrical objects that are coaxial. The rotor has an outer diameter that is less than the inner diameter of the stator so that there is a gap between the inner diameter of the stator and the outer diameter of the rotor. Thus, rotation of the rotor relative to the stator should not cause contact between the rotor and stator. Rotation occurs on bearings that support the rotor within a cylindrical cavity of the stator. When the rotor rotates relative to the stator, an unbalanced rotor, unbalanced magnetic field creation, oil whirl in the bearings, i.e. where the lubricating oil wedge lifts the rotor out of axial alignment under light loading conditions, or magnetic pull, i.e. where magnetic forces between the rotor and the stator pull the rotor off of balance, may cause the rotor to vibrate during operation. The vibration may occur such that the rotor, or portions of the rotor are no longer coaxial with the stator.
The vibration may have varying amplitudes along the length of the rotor. Higher amplitudes of vibration occurring at the location of the rotor bearings is of particular concern. During operation, when the rotor vibrates relative to the stator, the bearing may pound against the inner diameter of the stator, damaging the insulation of the stator and potentially causing a short. It addition, the components (T-rings) that prevent rotation of the stationary component of the bearing may become damaged and allow the bearing to rotate inside of the stator. This too may damage stator insulation and cause a short. In some cases, the rotor may contact the stator, causing damage to the stator insulation and a short. Still further, if the vibration has a high enough amplitude, the vibration may overcome the loading capacity of the fluid film of the bearing. This will cause wear and erosion of the bearing that will generate heat that may also cause a short in the motor. Therefore, an improved technique for attenuation of motor vibration in an ESP are needed.